Semiconductor devices that have integrated circuits are produced by fabricating a large plurality of identical circuit patterns on a semiconductor wafer using photolithography in combination with various other processes. It is a continual goal of semiconductor manufacturers to increase the density of semiconductor devices fabricated on a given size of semiconductor substrate to achieve increased yield of semiconductor devices and enhanced performance thereof. In recent years, efforts to increase the density of semiconductor devices in a semiconductor assembly have also intensified. One way to increase the density of semiconductor devices in a semiconductor assembly is to stack semiconductor dies upon one another. The semiconductor dies are interconnected by forming vias or through holes in the semiconductor dies. The vias are filled with an electrically conductive material to electrically connect the vias to integrated circuitry fabricated on an active surface of the semiconductor die. Thus, the vias provide a conductive pathway from the active surface of the semiconductor die to its respective back surface, enabling interconnection of the back surface of the semiconductor die to external electrical contacts of another semiconductor die or a carrier substrate. The vias are formed by etching, laser ablation or drilling, or a combination thereof. Etching the vias utilizes photolithographic processing of a photoresist followed by wet (chemical) or dry (reactive ion) etching. Laser drilling has been used to form vias by ablating semiconductor material to form through holes extending through the entire thickness of a semiconductor die.
The vias electrically interconnect various metal interconnection structures on the semiconductor die, such as annular rings, bond pads, component leads, metal wires, or other metal layers, to one another. Bond pads on the semiconductor dies are typically formed from aluminum, copper, or aluminum-copper alloys having less than about 0.5% copper. Aluminum is used in bond pads because of its low resistivity, superior adhesion qualities, high thermal stability, and ease of workability. However, one disadvantage of aluminum is that it readily oxidizes to form aluminum oxides, which decrease quality of the electrical connection and the efficiency of the bond pads. To protect the bond pads, the aluminum is etched to remove the aluminum oxides and covered with a barrier metal, such as a nickel layer. Tungsten is also commonly used as an interconnection material because it has a thermal expansion coefficient that is similar to that of silicon and has a good filling capability in semiconductor structures with high aspect ratios. Tungsten is typically used to fill or line a surface of an opening produced during formation of a via. The tungsten lining in the via is subsequently covered with a metal layer, such as a nickel layer. The bond pads are typically nickel plated after vias have been plated with nickel because etchants used to remove the aluminum oxides also remove nickel plating from inside the vias.